Nucleus is present in all eukaryotic cell except in some mature cells like in HUMAN RBC it is absent. It is very important for the survival of cell without it the cell can not divide or differentiate.
STRUCTURE OF NUCLEUS:
Organelles present in nucleus are—– Nuclear envelop or karyotheca, nuclear sap or karyolymph, nuclear matrix where network of protein is suspended , Fine intermingled nucleoprotein filament called chromatin , one or more rounded body called nucleoli.
Chemical composition—- 80 % protein, 12% DNA, 5% RNA ,& 3% lipids. Protein include enzymes like polymerase which help in DNA & RNA synthesis like wise the minerals like Ca , Na, K, Mg , are also present.
NUCLEAR ENVELOP —- It is a double membrane structure inner and outer one ,the outer membrane is rough because of the presence of ribosomes on it and it is continuous with endoplasmic reticulum at many places this will help it to shrink and expand by taking or giving the material. The inner membrane is smooth and having protein matrix and chromatin close to it hence chromosomes are at the periphery near it. The nuclear envelop is perforated by small pores which allow selective passage of material from outside to inside and vice versa. These are called nuclear pores.
Nuclear pores—–Each nuclear pore is fitted with pore complex which is a protein structure ,it projects into both cytoplasm and nucleoplasm . Pore complex consists of two rings the annuli each having 8 subunits and sending spoke into the pore which encloses channel.The passage of macromolecules such as tRNA , mRNA, ribosome proteins , nucleotides, enzymes , and ribosemes subunits take place through it,because they can not pas through lipid bilayer of nuclear envelop.
Nuclear envelop formed during teolophase.
Nuclear envelop disappears during short period during cell division.
FUNCTIONS—– Maintain the shape of nucleus, allow the in and out movement of material, keeps the nuclear content distinct from cytoplasm, protects the genetic material from the respiratory enzymes present in cytoplasm.
KARYOLYMPF OR NUCLEOPLASM—– Clear nonstaining fluid contain nucleotides polymerase enzyme, histone non histone priteins & ions Mg , Mn for DNA RNA synthesis.
NUCLEAR MATRIX —– Network of fine crisscross protein fibrils which are joined to the nuclear envelop by their ends .We can say that they form nuclear skeleton.